1. Field of the Invention
The present invention concerns the fabrication of film capacitors. It pertains more particularly to the fabrication of stacked or coiled capacitors capable of undergoing severe electrical treatment without causing any loss in capacitance.
2. Description of the Prior Art
The fabrication of stacked capacitors using metallized plastic films is known. Generally, plastic films with a metallized zone and a non-metallized lateral margin are used. Two films are superimposed, with their non-metallized lateral margins on opposite sides, in order to obtain a pair of films formed by an odd-order film and an even-order film. At least one pair of metallized films is coiled on a large-diameter wheel in a number of determined turns. A capacitive strip with alternating even-order and odd-order layers, called a parent capacitor, is obtained. Each of the lateral faces of the capacitive strip is then coated with a metal in order to make plates. Each plate enables the metallizations of the layers having the same order to be electrically and mechanically connected together. This operation is known as schoop-plating. By cutting out the capacitive strip into blocks, distinct capacitors of a generally parallelepided shape are obtained They are formed by a stacking of alternative even-order and odd-order sheets, held between two plates located on the opposite faces of the block.
Connection elements are then fixed to the plates of each capacitor.
Coiled capacitors, for their part, are generally fabricated individually. As above, at least one pair of metallized films is used, coiled on itself. At the end of a number of turns, a cylindrical unit is obtained. Metallic plates are deposited on the two lateral faces of the block by schoop plating. These faces are then circular. Then, a connection element is fixed to each of the plates.
The plastic films used are generally made of polyamide, polyester etc. Their thickness ranges from some micrometers to some tens of micrometers. During the metallization, defects may appear on the film: it may get pierced, dust may get attached to it, etc. A known way of getting rid of these defects is to make the finished capacitors undergo severe electrical treatment. It suffices to apply a high voltage to the connection elements. This treatment attacks all the defects in the film at the same time, by the phenomenon known as "self-healing" or "clearing". Through this phenomenon, high currents are induced on each defect, but these high currents also go through the plates and, in particular, the connection between a plate and a metallized zone of the film. When the current is too high, it can happen that the connection will act like a fuse and the capacitor then loses a part of its capacitance corresponding to the capacitive sheet touched by the fused connection
The present invention seeks to overcome this drawback and enables the electrical treatment of film capacitors of the stacked or coiled type, in inducing very high currents which, in normal times, would have destroyed all their connections.